BT412/context/locomotion.md

---
id: locomotion
title: "Locomotion — gait, the ground model, collision, shadow"
status: established
source_sections: "PROGRESS_LOG.md §7, §10 (gait/ground/collision/shadow); docs/P3_LOCOMOTION.md"
related_topics: [asset-formats, decomp-reference, reconstruction-gotchas, combat-damage, rendering]
key_terms: [RootTranslation, ground-model, SequenceController, MechControlsMapper, SLD, joint]
open_questions:
  - "Authentic per-mech turn-rate constant (currently bring-up constant; the authored WalkingTurnRate/RunningTurnRate now STREAM correctly since the task #4 record-layout fix -- rec words 0x18/0x19 deg->rad -> mech+0x574/0x578, Wword-absorbed -- wiring them in retires the bring-up rate)"
  - "Airborne clip callbacks (FUN_004a7970) deferred"
---

# Locomotion

Walking, the gait controller, the ground/collision model, and the shadow. All DEFAULT-ON now
(`BT_GAIT_CUTOVER`, `BT_GAIT_SM`, `BT_COLLISION`, `BT_REAL_CONTROLS` — set `=0` to fall back). Full
detail: `docs/P3_LOCOMOTION.md` + `docs/PROGRESS_LOG.md §7, §10`.

## Walk speed is animation-driven (not physics)
Each `.ANI` has a `[RootTranslation]` where **`.z` = forward root SPEED** (world units/s). The
engine `AnimationInstance::Animate(dt, move_joints)` integrates `movement += Δt·rootTrans.z` and
moves the body that far → feet plant by construction (no stride constant; k=1). Idle anims have
z=0. The `.MOD` physics governed thrust VEHICLES, NOT walk gait. [T1]

## The MARCHING-GHOST desync — ROOT-CAUSED + FIXED (2026-07-13, live diagnosis) [T2]
The long-parked "replicant walks in place while its master stands still" reproduced live with
telemetry running: the master's `[gaitSM]` showed `state=0 cycleSpeed=-2.50725 kfCur=19` — BOTH
channels stuck at a REVERSE cadence inside Standing.  Mechanism: the walk-family stop gate
(`cycleSpeed <= ZeroSpeed`, mech2.cpp) passes trivially for NEGATIVE (reverse) cadences, and
several stand-entry paths (turn exit, terminal poses) never touch the cycle — so a reverse-stop
enters Standing at full reverse cycle speed.  The master looks still (case-0 never advances the
clip; `kfCur` freezes) but the stale cycle REPLICATES, and the peer's replicant animates it =
the marching ghost.  A clean FORWARD stop decays the cycle first (`legSum ~3e-8` on the healthy
node) — which is why the bug was intermittent and direction-dependent.  FIX: all four
Advance*Animation Standing cases zero their channel's cycle on entry + ForceUpdate(8) so the
peer hears the stop immediately.  (The signed-demand change (task #15) is what let negative
cycles reach machinery written for forward decel.)  The earlier "bob reduction + stutter"
symptom family likely shares this root — re-observe after this fix before chasing separately.

## The two-channel gait (real controls)
The BT `SequenceController` (`seqctl.cpp`, from `SelectSequence@004277a8`+`Advance@0042790c`) drives
locomotion. Under `BT_REAL_CONTROLS` two channels run — **roles corrected 2026-07-09 (task #49) from
raw disasm** (both Advance fns had NO static decomp caller; found by byte-scanning for `e8` calls):
- **LEG channel = the LOCAL sim.** The master perf (the `0x4a9b5c` gap region) calls
  `AdvanceLegAnimation` @0x4aa399 (airborne @0x4aa388) and stores `-dist/dt` into **localVelocity
  (+0x1cc)** — the leg drives the local mech's TRAVEL and, advancing LAST (mj=1), the DISPLAYED pose.
  It reads the LIVE `speedDemand` and owns trn/turn-in-place + the early-stand wind-down guard. [T1]
- **BODY channel = the replication PROJECTOR.** `IntegrateMotion` (0x4ab1c8; body advance @0x4ab312,
  from `FUN_004ab430`, the projected-origin/dead-reckon updater) stores `-dist/dt` into
  **projectedVelocity (+0x2a0)** — the dead-reckoning feed for update records. Its phase drift is
  locally INVISIBLE in the binary. The literal projectedVelocity store is unwired, but the
  update-record velocity feed is live via the localVelocity/worldLinearVelocity maintenance
  (mech4.cpp VELOCITY STORAGE block) — replicant gait (task #50) runs off it. [T1]
**v5 (current): display == travel BY CONSTRUCTION through the LEG** (body advances first, leg last).
History: v3 (travel=leg, display=body) foot-slipped — two channels, different consumers; v4 unified
BOTH on the body (mis-reading 0x4ab430 as the travel source) — feet planted, but whenever the two
state machines phase-split, the out-of-phase LEG pose showed through on frames the body didn't write
(Standing/wind-down) = the RECURRING "legs stutter / lose sync" report. The split mechanism [T2,
reproduced via `BT_FORCE_OSC` + `BT_FORCE_TURN` + `BT_SYNC_LOG`]: (a) SEED — the bring-up trn trigger
arms only the leg (authentic trn dispatcher still unlocated), so the walk re-entry lands 1 frame
apart; (b) AMPLIFIER — with offset clips the end-of-clip callbacks fire on different frames, so a
demand change landing between them picks DIFFERENT next states (one winds down, one keeps walking)
→ opposite-phase churn. Under v5 both effects still exist but only skew the projection channel. [T2]
The gait STATE MACHINE (`AdvanceBodyAnimation`, mech2.cpp) slews `bodyCycleSpeed` toward the demand
through walk/run caps (`LoadLocomotionClips`, mech3.cpp:326 — measures `walkStride`/`standSpeed` via
`legAnimation.SelectSequence`). Transition callbacks (`Mech::BodyClipFinished @0x4a6d8c`,
`LegClipFinished @0x4a6928` — PE-parsed jump tables) alternate walk 6↔7 / run 12↔13 + walk→run. [T2]
⚠ **Aliasing bug class (namedClip):** `namedClip[]`@0x5e0 == `animationClips[]`@0x5cc+0x14 (ONE array);
`gimpBaseClip`@0x64c == `animationClips[0x20]`; `bodyAnimationState`@0x728 == `bodyStateAlarm.level`.
Declared-separate members don't see each other's writes — alias them.
Replicants (task #50): travel = engine DeadReckon; leg channel driven joints-only from the replicated
worldLinearVelocity -> speedDemand (mech4.cpp REPLICANT MOTION) — detail in [[multiplayer]]. [T2]

## The AUTHENTIC 1995 ground model (default-on, `BT_GROUND_REAL`)
Decoded by the 10-agent `ground-model-decode` workflow (binary `FUN_004a9b5c` = the master perf).
**NO GRAVITY anywhere** — accelerations zeroed every frame. Vertical position = an ABSOLUTE per-frame
ground SNAP:
1. ctor lifts `collisionTemplate->minY` by 5% of volume X-width (the probe height + wall-vs-floor separation).
2. per frame `MoveCollisionVolume()` re-finds the containing node in the zone's **BoundingBoxTree**
   (BOXTREE.cpp — NOT a heightfield).
3. probe `q = origin + (0, lift, 0)` → `FindBoundingBoxUnder(q, &h)` (h = distance down to the highest
   solid top; −1 = miss).
4. snap gate `h > 1e-4`: `origin.y -= (h − lift)` ⇒ y = surfaceY exactly. **MISS ⇒ NOTHING** (y holds,
   no runaway). Up-slope bounded by the lift window (implicit step allowance); walk-offs drop instantly.
5. `GetCurrentCollisions` → `ProcessCollisionList` → `Mech::ProcessCollision` (@0x4abb40, vtbl+0x3c):
   BoxedSolid resolver → StaticBounce → owner classification. Crushable icon (flags&0x8000) = damage
   0.00123f sentinel = move stands; damage>0 = **FULL FRAME REJECTION** (walls block, never slide);
   impactVel²>40 = crash-anim (SetLegAnimation(0x20)). [T2]
KEY: `FindSmallestNodeContainingColumn`/`FindBoundingBoxUnder` (`FUN_0040e36c`/`0040e5f0`, BOXTREE.CPP)
are ALIVE in the 2007 engine — the "heightfield" labels were wrong (caused two reverted attempts). A
master mech needs a **CollisionAssistant** (GetCurrentCollisions iterates it unchecked). BT solids
ship ZERO tiles (h never negative). [T2]

## Knockdown / crash + the desync/stutter fixes
Wall impacts iv²>40 (~6.3 u/s) bind the `bmp` stagger clip. Two documented bug fixes: (a) the
knockdown must stagger BOTH channels (`SetBodyAnimation(0x20)` + `SetLegAnimation(0x20)`) or display
+ travel split permanently (foot-slip); (b) a contact-hysteresis gate (0.4 s `gBlockCooldown`)
stops the knockdown re-firing on sustained/glancing contact. `localVelocity` is zeroed while crashed
(spec-faithful) so a knocked mech can't keep advancing. [T2]

## Controls (`BT_REAL_CONTROLS`, default-on)
`MechControlsMapper` (mechmppr.cpp @004afbe0; btl4mppr.cpp mappers) interprets input → `speedDemand`
/ `turnDemand`. ⚠ **WndProc NEVER receives WM_KEYUP** (the engine's per-frame reader `GetMessage`s
them out) → poll `GetAsyncKeyState` per frame, gated on foreground. Virtual controls: W/S sweep a
persistent throttle LEVER with a zero detent; A/D a momentary auto-centering stick. [T2]

## Shadow (default-on, `BT_SHADOW_TILT`)
BT 4.10's shadow is the `*_tshd.bgf` proxy, posed by `jointshadow` (terrain tilt — the SKL's own
contract: "apply terrain angle to pitch and roll") + `jointtshadow` (torso twist). The working 2026-07-09
arrangement (user-verified live: feet layer over it, opaque, survives inclines/declines) has FOUR
co-dependent parts — change one, re-check the others:
1. **TILT** (mech.cpp `UpdateShadowJoint`): quad up aligned to the surface normal via an orthonormal
   basis fed to the ENGINE's own LinearMatrix→EulerAngles conversion — hand-derived Euler signs are
   exactly how two prior tilt attempts "dug into the hillside"/vanished. ~35° cliff-guard cap.
2. **SAMPLER** (mech4.cpp): gradient from the collision probes PLUS `BTVisualGroundLift` per probe
   (the quad hugs the VISIBLE terrain, whose lift varies across a slope); world→local by the TRUE yaw
   from `localToWorld` (NOT the `gDriveHeading` scalar mirror — the task-#48 drift bug class).
   `BT_SHADOW_LOG` traces the normals.
3. **DRAW ORDER** (`PASS_SHADOW`, l4d3d.h/L4VIDRND/L4VIDEO): shadows draw between the STATIC terrain
   and the DYNAMIC opaque bodies (classic decal order), so the mech z-passes over the shadow — the
   bias can never paint the shadow OVER the feet, structurally.
4. **BIAS pairing** (L4D3D.cpp): tilt on (default) → decal epsilon −0.0008; `BT_SHADOW_TILT=0`
   (flat A/B fallback) → the old −0.004 it needs to survive slope burial. `BT_SHADOW_BIAS` overrides.
5. **LOADER-LEVEL TAG** (L4D3D.cpp `LoadObjectBGF`): `*tshd*` filenames get `SetIsShadow(1)` in the
   LOADER, not at call sites. The V VIEW TOGGLE (`SetViewInside`) and the damage swaps RELOAD segment
   meshes; an untagged reload disengages the whole pipeline — solid opaque black, no bias, buried on
   any slope ("shadow broke mid-session after pressing V", user-confirmed fixed). The all-shadow-
   material detector misses the mech proxy (plain black material, not shadow_mtl). [T2]
Polar maps have STUB `*_tshd` (shadow authored-off on snow). The visual-ground conform (`btvisgnd.cpp`)
stays render-only (never touches localOrigin). [T2]
⚠ **"Shadow vanishes on inclines" now has THREE confirmed/suspected causes — diagnose before fixing:**
(a) the V-toggle/reload UNTAG above (solid black + buried; broke mid-session; the likeliest cause of
the recurring reports — a relaunch always "fixed" it because the fresh build re-tagged); (b) gait
desync popping the displayed root pose past the bias margin (the first 2026-07-09 recurrence co-
resolved with gait v5, no shadow code changed — though (a) may have contributed) [T2 obs; mechanism
T4]; (c) genuine slope burial when the quad is flat/mis-tilted (the depth-bias margin is finite).
Check `[shadowobj]` tag lines and `[sync]`/`BT_SYNC_LOG` before touching bias/tilt.

## Key Relationships
- Detail: `docs/P3_LOCOMOTION.md`. Uses: [[asset-formats]] (SKL/ANI), [[decomp-reference]] (offsets).
- Feeds: [[combat-damage]] (collision→damage), [[rendering]] (shadow/visual-conform).